Atomic Structure
Atomic Theory
The Greeks
History of the Atom
 In 400 B.C the Greeks tried to
understand matter (chemicals)
and broke them down into
earth, wind, fire, and air.
~
~
Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
 Greek philosopher
 Idea of ‘democracy’
 Idea of ‘atomos’
 Atomos = ‘indivisible’
 ‘Atom’ is derived
 No experiments to support idea
 Continuous vs. discontinuous
theory of matter
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Four Element Theory
FIRE
 Plato was an atomist
 Thought all matter was
Hot
composed of 4 elements:
 Earth (cool, heavy)
 Water (wet)
 Fire (hot)
 Air (light)
AIR
Dry
EARTH
‘MATTER’
Wet
Cold
 Ether (close to heaven)
WATER
Relation of the four elements and the four qualities
Blend these “elements” in different proportions to get all substances
Some Early Ideas on Matter
Anaxagoras
(Greek, born 500 B.C.)
–Suggested every substance had its own kind of “seeds” that clustered together to
make the substance, much as our atoms cluster to make molecules.
Empedocles
(Greek, born in Sicily, 490 B.C.)
–Suggested there were only four basic seeds – earth, air, fire, and water. The
elementary substances (atoms to us) combined in various ways to make everything.
Democritus
(Thracian, born 470 B.C.)
–Actually proposed the word atom (indivisible) because he believed that all matter
consisted of such tiny units with voids between, an idea quite similar to our own
beliefs. It was rejected by Aristotle and thus lost for 2000 years.
Aristotle
(Greek, born 384 B.C.)
–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements which
combined as shown in the diagram (previous page).
Hot + dry made fire; hot + wet made air, and so on.
O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,
Alchemy
 After that, chemistry was
ruled by alchemy.
 They believed that that
could take any cheap metals
and turn them into gold.
 Alchemists were almost like
magicians.
 elixirs, physical immortality
Alchemy
Alchemical symbols for substances…
..
.
......
.
.....
GOLD
SILVER
COPPER
IRON
SAND
transmutation: changing one substance into another
D
In ordinary chemistry, we cannot transmute elements.
Contributions
of alchemists:
Information about elements
- the elements mercury, sulfur, and antimony were discovered
- properties of some elements
Develop lab apparatus / procedures / experimental techniques
- alchemists learned how to prepare acids.
- developed several alloys
- new glassware
The Atomic Theory of Matter
 In 1803, Dalton proposed that elements consist of individual
particles called atoms.
 His atomic theory of matter contains four hypotheses:
1. All matter is composed of tiny particles called atoms.
2. All atoms of an element are identical in mass and
fundamental chemical properties.
3. A chemical compound is a substance that always
contains the same atoms in the same ratio.
4. In chemical reactions, atoms from one or more
compounds or elements redistribute or rearrange in
relation to other atoms to form one or more new
compounds. Atoms themselves do not undergo a
change of identity in chemical reactions.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
The Atomic Theory of Matter
 Dalton’s atomic theory is essentially correct, with four
minor modifications:
1. Not all atoms of an element must have precisely the same mass.
2. Atoms of one element can be transformed into another through
nuclear reactions.
3. The composition of many solid compounds are somewhat
variable.
4. Under certain circumstances, some atoms can be divided
(split into smaller particles: i.e. nuclear fission).
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Dalton’s Symbols
John Dalton
1808
Subatomic particles
 Electrons:
 Negatively charged subatomic particles
 Discovered by J.J. Thomson
 Discovered by observing deflection of cathode rays.
National High Magnetic Field Laboratory:
Electromagnetic Deflection in a Cathode Ray Tube (I)
Tutorial
 Protons : positively charge particles
 Neutrons: no charge particles, mass similar to
protons.
 The atomic nucleus:
 Protons and neutrons are located in the center of the
atom
 Through the gold-foil experiment, Rutherford
determined: YouTube - Rutherford's Experiment:
Nuclear Atom


The atom is mostly empty space
Positive charge and most of atom’s mass is concentrated in a
small region, called the nucleus (composed of protons and
neutrons)
Distinguishing Among Atoms
 Elements are different because they contain different
number of protons.
 Atomic number: indicates the number of protons in
the nucleus of an element.
 Since atoms are electronically neutral:
# protons= # electrons
6
C
Atomic number:
Carbon has 6 protons
Carbon has 6 electrons
Electron Microscopy
 Images of objects are produced by using an electron
beam.
 Typical light microscope magnifies up to 1000 x
 Electron microscope magnifies over 100,000 x
 SEM - Image Gallery
Learning Check: complete the
following table
Element
1.
Symbol
Atomic
number
# protons
Tin
2.
16
3.
81
4.
5.
# electrons
76
Gd
Learning Check: complete the
following table
Element
Symbol Atomic #
number protons
#
electrons
1.
Tin
Sn
50
50
50
2.
Sulfur
S
16
16
16
3.
Thallium
Tl
81
81
81
4.
Osmium
Os
76
76
76
5.
Gadolinium
Gd
64
64
64
Mass number
 Most of the mass of an atom is concentrated in its
nucleus and depends on the number of protons and
neutrons.
 Mass number: total number of protons and neutrons
in an atom.
 # neutrons= mass number – atomic number
 Representing atoms:
Mass number
197
Atomic
number
79
Au
Mass number
gold-197
Learning check: determine the
number of protons, neutrons and
electrons for the following atoms.
1.
Carbon-12
2. Fluorine-19
3. Beryllium-9
Learning check: determine the
number of protons, neutrons and
electrons for the following atoms.
1.
Carbon-12 p+= 6 e-=6 no= 6
2. Fluorine-19 p+= 9
e-=9 no= 10
3. Beryllium-9 p+= 4
e-=4 no= 5
4.
200
80
Hg
5.
207
82
Pb
4.
200
80
Hg
p+= 80 e-=80 no= 120
5.
207
82
Pb
p+= 82 e-=82 no= 125
Isotopes
 Isotopes are atoms of the same element that have the
same number of protons, but different number of
neutrons.
 Isotopes of an element have the same atomic number
but different number of neutrons, thus have different
mass numbers.
 Hydrogen has 3 isotopes:
 Hydrogen-1 or simply hydrogen
 Hydrogen-2 or deuterium
 Hydrogen-3 or tritium
Learning check: write the symbol
(including atomic number and mass
number )for the following isotopes.
1.
oxygen-17, oxygen-18
2. Chromium-50, chromium-52, chromium-53
Learning check: write the symbol
(including atomic number and mass
number )for the following isotopes.
1.
oxygen-17, oxygen-18
17
18
O
8
8
O
2. Chromium-50, chromium-52, chromium-53
50
Cr
24
52
24
Cr
53
24
Cr
Atomic Mass
 Mass of proton or neutron: 1.67x10-24g
 Mass of electron :
9.11x10-28g
 These values are impractical to work with, so scientists
compare relative masses of atoms using a reference
isotope : carbon-12
 An atomic mass unit (amu) is defined as 1/12 the
mass of carbon-12
Atomic mass (continued)
 The atomic mass of an element is not a whole number
because the isotopes of an element and its natural
abundance is taken in consideration.
 The atomic mass of an element is a weighted average
mass of the atoms in a naturally occurring sample of
the element.
Atomic mass (continued)
 Ex. 1 The atomic mass of copper is 63.546 amu.
Which of copper’s two isotopes is more abundant:
copper-63 or copper-65?
 Since 63.546 is closer to 63 than 65, the most abundant
isotope is copper-63.
Atomic mass (continued)
 Ex. 3 There are 3 isotopes of silicon; they have mass
numbers of 28, 29, and 30. The atomic mass of silicon
is 28.086 amu. Comment on the relative abundance of
these 3 isotopes.
 Silicon-28 must be the most abundant.
Atomic mass (continued)
 Ex. 2 Calculate the atomic mass of bromine. The two
isotopes of bromine have atomic masses and relative
abundance of 78.92 amu (50.69%) and 80.92 amu
(49.31%)
1. Divide the percentages by 100: 0.5069 and 0.4931
atomic mass= (0.5069x78.92) + (0.4931x80.92)
=79.9062
= 79.91 amu (remember units and sig figs)
Atomic mass (continued)
 Classwork p103 # 12,13; p 105 #33-34